The art of vehicle detection by drive-up loops is well known and has gained widespread recognition by drive-up businesses, such as fast food restaurants, drive-up bank teller systems and others. As representative examples of loop detectors, reference may be made to the following U.S. Pat. Nos. 2,601,472; 3,164,802; 3,389,717; 3,530,434; 3,564,219; 3,581,068; 3,609,247; 3,614,729; 3,651,452; 3,652,832; 3,663,938; 3,665,382; 3,685,013; 3,818,430; 3,820,100; 3,868,626; 3,873,964; 3,875,555; 3,900,829; 3,942,108; 3,943,339; 3,980,867; 3,989,932; 3,996,555; 4,038,633; 4,131,848; 4,201,908; 4,231,114; 4,247,947; 4,251,797; 4,296,401; 4,311,876; 4,352,183; 4,358,749; 4,391,119; 4,392,119; 4,430,636; 4,471,356; 4,472,706; 4,491,841; 4,529,982; 4,568,937; 4,630,044; 4,639,689; 4,668,951; 4,680,717; 4,714,925; 4,731,867; 4,754,277; 4,862,162; 4,873,494; 4,920,340; 4,926,494; 4,996,716; 5,028,921; 5,089,815; 5,153,525; 5,184,350; 5,198,811; 5,220,677; 5,247,297; 5,278,555; and 5,281,965.
In such systems, a vehicle drives up or across a designated detection area, such as a remote order taking station for placing a food or drink order without entering a fast food restaurant. A conductive wire loop embedded in the detection area has a given inductance which changes as a result of the vehicle coming in close proximity to the inductive loop.
This change in inductance, in turn, causes a loop detection system to generate an alarm signal which alerts an order taker that a customer is present at the remote order taking station.
While such systems have been satisfactory for certain applications, they have not always proven to be entirely reliable. In this regard, the amount of the inductance change associated with any given inductive loop is dependent upon the size and shape of the vehicle crossing the loop, the metallic content of the vehicle, the depth the loop is embedded within the surface, the size and shape of the conductive loop, and the effects of climatic conditions, such as rain, snow, humidity, and other similar conditions.
The aforesaid lack of reliability of the prior known systems is due to the resonance sensitivity of the detection circuits which sense the changes in inductance in the loop. In this regard, such detection circuits have not been capable of reliably sensing relatively small inductance changes in the loop.
One possible solution to the inherent lack of sensitivity is disclosed in U.S. Pat. No. 3,875,555, which describes an oscillation detection circuit utilizing a direct frequency measurement approach, rather than measuring a phase shift change or amplitude change in the output of the system oscillator as its tank circuit is mistuned out of resonance. Such an approach has not been entirely satisfactory. In this regard, the disclosed prior art system is very expensive as it required the use of not only two oscillators, a reference oscillator and a loop oscillator, but also require expensive tracking circuits to compensate for environmental drift conditions.
Another problem associated with the reliability of such prior art systems is the difficulty in tuning the reference frequency oscillator with the loop oscillator to compensate for the environmental drift conditions. In this regard, in order to detect properly a frequency difference between the two oscillators, they must initially be tuned to the same frequency. However, because the output frequency of the loop oscillator is a function of the size and shape of the conductive loop, the depth at which the loop is embedded within the surface, and the environmental conditions at the time of installation, it is difficult, if not impossible, to tune the two oscillators to substantially the same frequency.
Therefore, it would be highly desirable to have a vehicle detection system suitable for use at a variety of drive-up stations having different size and shape inductive loops, under a variety of weather conditions. Such a system should enable the detection of a motor vehicle in a reliable manner, even though the loops may be embedded at different depth in their associated drive-up lanes. Moreover, such a system should be convenient to use and install and not be unduly expensive to manufacture.